Hard disk drives utilize a pivotable actuator with magnetic heads to read information from and to write information to magnetic disks. As shown in FIG. 1, the actuator 11 is typically mounted to a motor-driven pivot shaft 13 which rotates the heads on actuator 11 to precise locations on the disks (not shown). Shaft 13 is surrounded by a stationary housing 15. At least one pivot bearing 17 is mounted between housing 15 and shaft 13 to enhance the stability of shaft 13 as it pivots actuator 11. During operation, bearing 17 emits oil vapor and aerosol droplets of grease which are a potential source of contamination in the disk drive. Reducing this type of contamination is important to improve the reliability of disk drives.
Prior art disk drives have used several different types of seals to seal the actuator pivot bearings in disk drives. One type of bearing seal, known as a ferrofluid seal, is practically impermeable to emissions from bearings and is fairly well known in the art. Ferrofluids contain very small magnetic particles which are suspended in a carrier fluid. Ferrofluid seal 21 (FIGS. 1 and 2) utilizes shaft 13 which is formed from a magnetic material, such as 400 series martensitic stainless steel, and a magnet 23 mounted to housing 15. Magnet 23 is spaced apart from shaft 13 by a very small gap 25. Ferrofluid seal 21 functions by suspending a magnetically attracted fluid 27 in gap 25 between magnet 23 and shaft 13 to complete the magnetic circuit (see magnetic flux lines 29 in FIG. 2). The suspended fluid 27, which is essentially frictionless, forms an effective liquid seal between the pivoting shaft 13 and housing 15.
Recently, non-magnetic materials for disk drives have become increasingly popular with disk drive designers for their ability to match the coefficient of thermal expansion of the actuator. Unfortunately, disk drives which utilize nonmagnetic actuator shafts are incompatible with conventional ferrofluid seals. A ferrofluid bearing seal for disk drives with nonmagnetic elements is needed.